I've been fascinated with poisonous plants since I was about 10 or 11 which I know sounds a bit ominous and if I tell you I had been really interested in witch craft since younger than that it probably doesn't help either.
My interest continued to grow when I started to read Agatha Christie books as a teenager, I was fascinated by the easy access to poisonous substances people had in the 1930s.
Now as an adult that fascination has gone away. The fact that growing along a river bank or at the bottom of your garden are plants that mimic death by heart attack, or provide a slow and painful death to which there is no antidote seems utterly bizarre.
There is a poison garden in Alnwick (https://www.alnwickgarden.com) that's been on my places to visit list for ages, and hopefully next year I'll finally fulfil that wish.
Finding ways to kills off characters is something I spend a lot of time thinking about, obviously there's being stabbed, strangled or shot (although less common in the UK) but poisoning opens up so many more interesting avenues, you can start asking questions like; how long has it been going on for, was it accidental, did someone else drink the orange juice meant for the vicar.
Then you can think about accessibility, because it's not just deadly nightshade and belladonna types of plants, you can also have alcohol poisoning, nicotine poisoning - like what if the victim was strapped down and covered in nicotine patches - how many would it take to kill them and how long would they need to be applied for. (Actually that one's pretty interesting perhaps I'll use it one day)
Poisoning opens up the world of murder for those that don't want to get their hands dirty, or at least bloody, it can be done at a distance when the killer has secured themselves an airtight alibi, or it can be cruel administered every day with the perpetrator watching the pain they inflict with morbid satisfaction.
The point is that there's a lot of ways to write a murder and poison is a lot of fun (as a writer only) When I'm choosing the method of murder I'm also thinking about what it says about the killer and these are the thoughts that shape characters and story lines.